There are many applications of threaded fasteners which require a so-called "locking fastener"; as used herein the term "locking fastener" means a threaded fastener which includes means for inhibiting unwanted loosening of the fastener to a greater extent than that which results from the friction which resists turning of one of the threaded members relative to the other in the loosening direction. In many applications of locking fasteners, it is also required that the fastener can be purposely loosened and removed and reinstalled without degradation of the parts.
There have been many proposals in the prior art to meet such requirements in threaded fasteners by using wedging action which causes interference or jamming of the threads in response to unwanted rotation of one of the fastener members. In such devices, the thread jamming is usually achieved by providing a ramp surface with a greater angle than the angle of the threads so that loosening rotation of the nut, for example, will cause it to back off axially at a faster rate than that permitted by the threads. This kind of coaction between wedging surfaces and fastener threads has been proposed to be accomplished by special washers with ramp surfaces. Also, the prior art includes proposals to incorporate the wedging surfaces in the structure of the fastener nut.
The use of wedging surfaces on washers is described in Trimble et al U.S. Pat. No. 428,183 granted May 20, 1890, Terry U.S. Pat. No. 2,665,729 granted Jan. 12, 1954 and Herpolsheimer U.S. Pat. No. 3,263,727 granted Aug. 2, 1966. In the Trimble et al patent and the Herpolsheimer patent, a conventional bolt and nut are used with a pair of special washers disposed between the nut and the workpiece, the washers being non-split annular members. Both of these patents disclose teeth on the surface of the inner washer which engages the workpiece and teeth on the surface of the outer washer which engages the nut to provide a greater resistance to rotation than that between the engaging faces of the two washers. This arrangement is provided to ensure that any rotation of the nut in the loosening or unscrewing direction will cause the wedging surfaces to rotate relative to each other and jam the threads to provide locking action against loosening.
In Trimble et al one of the opposed washer faces is provided with plural ramp surfaces and the other opposed washer face is provided with an axial projection which engages the ramp surface. For removing the nut, the Trimble et al patent discloses a notch in one of the washers at the end of the ramp to receive the projection whereby the combined thickness of the washers is reduced and the nut may be unscrewed without jamming of the threads.
In Herpolsheimer, the opposed washer surfaces are provided with opposed engaging ramp surfaces. The Herpolsheimer patent discloses that the nut may be removed by applying sufficient loosening torque to the nut to cause the ramp surfaces of the respective washers to override each other, the elongation of the bolt resulting therefrom being less than the yield point of the bolt. Also, it is noted that in Herpolsheimer, the nut could be removed by holding it against rotation and turning the bolt head in the loosening direction without the washers causing jamming of the threads.
A locking fastener with a single piece washer incorporating ramp surfaces for wedging action is disclosed in the Terry Pat. No. 2,665,729 granted Jan. 12, 1954. This locking fastener utilizes a conventional bolt and nut with a split washer between the nut and workpiece, the washer having overlapping ends with confronting ramp surfaces. The inner face of the washer opposite the workpiece is provided with serrations adjacent the free end and the outer face is also provided with serrations adjacent the free end for engagement with the nut. When the nut is tightened, the serrations on the washer tend to bite into the workpiece to resist rotation thereof and the serrations on the outer surface of the washer tend to bite into the nut and move therewith during tightening so that the washer is enlarged circumferentially and the washer thickness at the overlapping ends is reduced to a minimum value. Terry describes this action as having the result of tensioning the washers so that the overlapping ends are urged to contract toward each other by the tension forces set up in the washer; further, the tension forces urge the free ends of the washer toward each other so that the confronting ramps develop wedging forces which result in jamming of the threads to prevent loosening. The Terry patent describes an arrangement to limit the locking effect and thus permit removal of the nut by torquing it in the loosening direction. This arrangement comprises an axial shoulder on each ramp which is abutted by the free end of the other ramp to limit the thickness of the washer at its free end when the nut is turned in the loosening direction.
The Terry Pat. No. 4,708,555 granted Nov. 24, 1987 discloses a locking fastener using a split washer with confronting surfaces similar to that in the afore-mentioned Terry patent; it also discloses a pair of washers with confronting ramp surfaces. The later Terry patent describes the use of spring means for yieldably resisting relative movement of the confronting ramp surfaces in the tightening direction for storing energy to bias the ramp surfaces in the loosening direction to thereby cause jamming of the threads to prevent loosening of the nut. In the disclosure of this Terry patent, serrations are provided on the inner surface of the washer which engages the workpiece and on the outer surface of the washer which engages the nut.
The prior art also includes a locking fastener of the wedging type in which ramp surfaces are provided on the nut itself which is used in conjunction with a special washer. Such a locking fastener is disclosed in the above-mentioned Herpolsheimer Pat. No. 3,263,727 and the Terry Pat. No. 4,708,555. The Herpolsheimer patent discloses a bolt head having ramp surfaces which coact with confronting ramp surfaces on a special washer. The washer is provided with teeth which engage the workpiece to prevent rotation of the washer with the bolt head when it is turned in a loosening direction. The bolt is removed by applying sufficient torque to override the ramp surfaces which are of such dimension that the elastic limit of the bolt is not exceeded. The Terry Pat. No. 4,708,555 discloses a bolt having a nut provided with the ramp surfaces which confront corresponding ramp surfaces on a special washer. The nut in the Terry patent is removed by turning it and the washer simultaneously in the loosening direction to avoid the wedging action which would jam the threads.